The present invention is directed toward a doctor blade and, more particularly, to such a blade which is used in a chambered doctor blade system with end seals.
Chambered doctor blade inking systems employed in flexographic printing and Gravure coating often include elastomeric, elastomeric foam, felt or other deformable end seals. One function of the seals is to prevent ink from freely flowing out of the ends of the chamber, onto the rotating ends and journals of the engraved cylinder, where it would be sprayed tangentially by the rotation of the cylinder. Another function of the end seals is to allow static ink pressure above atmospheric pressure to be maintained inside the chamber in order to aid in the distribution and delivery of the ink to the engraved roll. A third function is to permit controlled ink leakage to provide lubrication and prevent excessive heating of the seals and blades. The controlled leakage is then intercepted and collected by other features of the design, all of which is well known in the art.
Chambered doctor blade inking systems normally include a pair of doctor blades and a seal at each end located between the doctor blades. The seals typically contact the wetted face of the blades and the segment of the engraved cylinder located between the two doctor blades.
With various types of ceramic-surfaced engraved cylinders, it is desirable to reduce friction and resulting doctor blade wear and/or roll surface damage by using doctor blades made of polyethylene, Teflon, or other plastic materials having a low coefficient of friction in contact with the ceramic surface. As these materials tend to have a relatively low tensile strength and modulus of elasticity, as well as relatively low dimensional stability, it is necessary to use thick blade body sections typically from between approximately 0.030 and 0.080 inch as opposed to the more conventional 0.006 to 0.008 inch sections of spring steel doctor blades. The working tip of the blade in contact with the engraved cylinder must, however, be small (approximately 0.001 to 0.005 inch) to conform to the cylinder with low applied force and to avoid unwanted hydraulic blade lifting effects.
These factors have resulted in a plastic doctor blade design cross section with a beveled tip incorporating an approximately 45 to 55 degree included plastic angle. However, this blade cross section can create end sealing problems making it necessary to add additional shaping to the seal to conform to the blade/cylinder interface. The required shape of the end seal at this interface is difficult to predict and/or achieve because of the complex interaction of blade deflection under applied actuating load, variable internal chamber pressure, and variable seal deformation.